It is already known in the prior art, for example, from DE-PS 934,407 to throw on to the impression cylinder the eccentric bushing mounted blanket cylinder of the printing unit of an offset press in two separate phases with a freely determinable interval between them. The blanket cylinder is first thrown on to the plate cylinder for pre-inking and only afterwards is thrown on to the impression cylinder. This is achieved by a cam-operated cam follower lever having a two-state pawl coupling. Throw-on and throw-off are effected when the grip edges of the cylinders are opposite one another. A disadvantage of arrangements such as these is the unfavorable mechanical dynamics, due to the harsh engagement of the pawl and the corresponding high driving torque which must be additionally provided by the main drive, particularly when the presses are running fast.
For two stage throw-on and throw-off of the blanket cylinder without the need for an abrupt input of torque from the main drive, it is known from DE 3,232,171 Al and from DD 86,631 to produce the movement of the eccentric bushings by means of double-acting, fluid pressure working cylinders adapted to operate consecutively. Disadvantages here, however, arise from the very high constructional complexity and the accompanying mass of the components which are associated with the blanket cylinder bearing levers, and the fact that the operation of the working cylinders corresponds to a pure series or consecutive arrangement so that the force evolved by one cylinder always reacts on the other cylinder cooperating with it.
In U.S. Pat. Ser. No. 3,067,674 a throw-on/throw-off device is disclosed having a fluid pressure actuating cylinder which acts on a toggle transmission linkage, the latter in turn pivoting the eccentric bushings of the blanket cylinder. A disadvantage in this case is that a three-point working cylinder is necessary for two-stage throw-on and throw-off. Such a cylinder corresponds essentially to a series arrangement of two double-acting working cylinders and needs to provide substantial forces for modern high-speed presses to ensure that the blanket cylinder separates fast enough from the impression cylinder at high printing speeds in the event a sheet to be printed is missing.
The related control systems for initiating throw on/off procedures for sheet-fed offset printing presses are generally known in the art. For example, references DE-AS 1 Q98 963, DD-PS 86 631 and DE 3 232 171 Al all disclose sheet-fed offset printing presses utilizing pressure-operated actuating means for throw-on and throw-off procedures. As disclosed in the above cited references, a blanket cylinder is placed in contact with a plate cylinder and/or an impression cylinder during each throw-on procedure, thus forming a nip at the point of contact, and the blanket cylinder is separated from the plate cylinder and/or the impression cylinder during the throw-off procedure.
The plate, blanket and impression cylinders all include grip edges for securing a printing plate, a transfer blanket for the ink, and a sheet of paper, respectively. During both throw-on and throw-off procedures, the blanket cylinder is placed in contact with or removed from the plate and impression cylinders one at a time, i.e., in a sequential order. A grip edge zone exists on the surface area of a cylinder, defined by the circumferential area between the grip edges that is not covered by the paper sheet, printing plate or blanket member. In operation, the grip edge zones of opposing cylinders are in contact or immediately adjacent to each other (if the corresponding cylinders are not in contact) during throw-on and throw-off procedures, and, therefore, are considered to be in "registration." This registration of the grip edge zones is necessary in order to ensure proper printing of each sheet. When printing is interrupted, the blanket cylinder is thrown off the plate cylinder after the last sheet is printed. The blanket cylinder is thrown off the impression cylinder in order to ensure that ink is not transferred to the impression cylinder in the event a sheet of paper is not present. Similar considerations apply to the throw-on procedure.
In conventional control systems, timers are used to activate the throw-on and throw-off procedures at a particular time and in a particular sequence. These timers are driven in synchronism with the press and include cams that control various changeover valves that regulate devices for actuating the throw-on or throw-off of the blanket cylinder. Throw-on or throw-off is generally initiated when the grip zones enter the nip between two cylinders. In order to ensure that throw-on and throw-off procedures occur at the end of printing (i.e., a time when the relevant grip edge zones are in registration), triggering signals are adjusted to occur as the grip edge zones enter the nip and towards the end of printing.
A disadvantage of an actuating time for throw-off and throw-on that is in a fixed relationship to press position and located inside the grip edge zones is that in the case of a high speed press there is only a very short time slot available between the time of initiating the actuating device and the time at which the blanket cylinder has been completely thrown on or thrown off the impression cylinder or plate cylinder. Following conventional designs, the actuating device would have to be designed to operate at very high speeds, and thus tolerate very high driving pressure forces in order to ensure that throw-on and throw-off could be completed entirely within the time during which the grip edge zone of the relevant cylinders are in registration. Such a design would be very expensive, and therefore, is undesirable.